Endotaxin activation of normal rat serum produces a factor(s) which is selectively chemotactic for T-lymphocytes by in vitro assay using Nelson's methods for quantitating cellular migration under agarose. This material differs from the complement leukotaxins (C'3a and C'5a) which are also generated by endotoxin serum treatment in that: the lymphocyte chemotactant effects are lost by heating activated sera for 1/2 hour at 56 degrees C but its activity persisted when serum from rats depleted of hemolytic complement activity by injections with cobra venom factor was incubated with endotoxin. This lymphocyte chemoattractant has an estimated molecular weight of greater than 70,000 by Amicon membrane filtration and elutes from DEAE cellulose columns in fractions corresponding to the albumin peak. Since highly purified albumin preparation cannot induce directional migration of lymphocytes, the active factor is postulated to be a low molecular weight material loosely bound to serum albumin. The partially purified material induces chemotactic responses in T-cells from thoracic duct lymph, con-A stimulated lymphoblasts and thymic lymphoma cells, but lacks demonstrable chemoattractant activity for granulocytes, macrophages, enriched B lymphocyte preparations, LPS-stimulated lymphoblasts and B-cell leukemia lines.